It's a closed system so it shouldn't need refilling unless there's a leak somewhere, e.g. where the lines connect to the pump or cylinders. However leaks are very uncommon. Not unheard of, but just not very common. Obviously, it it needs refilling, you need to investigate why and where it's leaking. Odds are it's not the hydraulics, but the roof flaps with a fault error. I've written extensively on the TT Mk2 Roadster roof, so you might want to drop by the Forum and say hello. :-) https://www.ttforum.co.uk/forum/

Having major problem with 2009 TTS, roof working intermittently. Been following all your threads. My only question is the hydraulic fluid a closed system in
this car? Or can and do you require to top it up.